1. Field of the Invention
The present invention relates to local area networks and, in particular, to an architecture for a coding and transceiving Ethernet circuit that connects a network interface controller to a transmission medium.
2. Description of the Related Art
A local area network is a communication system that allows personal computers, workstations, servers, and other network devices within a small area, such as a single building or a group of adjacent buildings, to transfer information between each other. Each device connected to the network communicates with other devices on the network by following a standard which defines the operation of the network. One of the most widely accepted standards for local area networks is the IEEE 802.3 CSMA/CD Ethernet Protocol.
In the CSMA/CD (Carrier Sense Multiple Access with Collision Detection) Ethernet Protocol, before transmitting data onto the network, each device first monitors the network to insure that no transmissions are currently in progress (Carrier Sense). When the network is idle (no transmissions in progress), each device can transmit information onto the network (Multiple Access). When more than one device transmits at the same time, each transmitting device must be able to detect this condition (Collision Detection), stop its transmission, and retry.
The IEEE 802.3 CSMA/CD Ethernet Protocol defines four physical layer specifications which differ primarily in the physical cables utilized. Coaxial cables are defined by a Thick Coax Ethernet (10BASE5), which utilizes a double-shielded coaxial cable, and a Thin Coax Ethernet (10BASE2), which utilizes a single-shielded coaxial cable. Twisted pair cables are 26 to 22 AWG unshielded wire. The Thick Coax Ethernet, Thin Coax Ethernet, and Twisted Pair Ethernet each utilize a 10 megabit per second data rate.
As shown in FIG. 1, an Ethernet node can be connected to an Ethernet network by first connecting a host bus of a network device to a network interface controller, such as the National Semiconductor Corporation DP8390 Network Interface Controller. The network interface controller implements all media access control layer functions in accordance with the IEEE 802.3 standard.
Next, the network interface controller is connected to a serial network interface, such as the National Semiconductor Corporation DP83910A CMOS Serial Network Interface. The serial network interface provides the Manchester data encoding and decoding functions specified in the IEEE 802.3 standard.
The serial network interface can then be connected to either a coaxial transceiver, known as a 10BASE5 medium attachment unit (10BASE5 MAU), or a twisted-pair transceiver, known as a 10BASE-T medium attachment unit (10BASE-T MAU) via an AUI (attachment unit interface) cable and isolation transformers. The coaxial transceiver can then be connected directly to a coaxial cable while the twisted pair transceiver can then be connected to a twisted pair media via an isolation transformer. The isolation transformers are utilized to electrically isolate the serial network interface from the coaxial transceiver and the twisted pair transceiver and to electrically isolate the twisted pair transceiver from the twisted pair media.
The 10BASE5 transceiver, such as the National Semiconductor Corporation DP8392 Coaxial Transceiver Interface, and the 10BASE-T transceiver, such as the National Semiconductor Corporation DP83922 Twisted Pair Transceiver (TPI), both provide transmitter, receiver, collision detection, jabber timer, and heartbeat functions in accordance with the IEEE 802.3 standard. In addition, the twisted pair transceiver provides a link test function and a link generation function.
In the installation shown in FIG. 1, the twisted-pair and coaxial transceivers are separate units which, as stated above, are connected to the serial network interface by an AUI cable. Frequently, in twisted pair applications, it is desirable to be able to connect an Ethernet node directly to a twisted pair cable and thereby eliminate the need for a separate twisted pair transceiver unit and a separate AUI cable. Thus, there is a need to combine the functionality of the twisted pair transceiver with the functionality of the serial network interface.